1. Field of the Invention
The present invention relates to coatings and methods for surface refinement, in particular for the production of permanent and stable surface coatings using inkjet printing methods.
2. The Prior Art
The term inkjet technology is understood below as meaning both inkjet and drop-on-demand methods. In particular, inks of any desired composition can be used in the drop-on-demand method. Inkjet technology is a digital printing technique that also makes it possible to produce very small print runs and/or batches down to a single item without an increase in the setup times. This technique is thus outstandingly suitable for the individualization of items.
It is, moreover, a non-impact technology, that is to say, the printing heads do not come into contact with the medium or substrate to be printed. This makes possible problem-free printing of a multiplicity of materials.
The use of the inkjet technology has hitherto been restricted, however, to media which make only small demands as such on mechanical and chemical stability and on resolution, or to media whose stabilities and resolution are improved by a suitable pretreatment, such as special inkjet papers, for example. Resolution is understood in this context as meaning the printed resolution as defined by the drop size and the positioning accuracy of the printing heads. Printed resolutions in the case of customary office printers (office sector) are in the range of up to 2880 dpi, and in the case of industrial applications (industrial sector) they are in the range of from 360 dpi to 1440 dpi.
The inks customarily employed in the inkjet printing method are also generally not adequately stable or resistant to make them suitable for applications in areas subject to high mechanical or physicochemical stresses, such as occur, for example, in the open.
EP 1 821 576 A1 and EP 1 860 923 A1 disclose methods for the production of electroluminescent display panels, in which images are applied to one side of a substrate and flat electrodes to the other side of the substrate by means of ink application implementing an inkjet printing method. To protect these prints against mechanical and chemical stresses, a further layer is laminated over the respective print.
Inkjet printing methods make high resolution and high printing speeds possible, which are moreover freely definable and infinitely variable. They are therefore also suitable for in-line decorating and refining processes. They can be adapted to the production flow, such that the production flow, more particularly the rate of production, can be maintained.
Restrictions in in-line production processes, however, result from the necessity to dry the inks employed. Printing with aqueous inks is therefore generally employed in the case of porous media, so that the coat applied hardens to an adequate extent before the next operation. With the use of solvent-containing inks, an appropriate drying zone is needed prior to the subsequent processing step, and such a drying zone is not always available in existing plants.
U.S. Pat. No. 4,906,170 discloses a method of marking pipes of plastics material by means of inkjet printers, in which a drying zone is inserted downstream of the printing unit. The drying zone is sufficiently long to avoid smudging of the print in the next step.
On account of the above drawbacks, especially as regards the stability factors, inkjet printing methods have not been employed hitherto in industrial processes for surface refinement.